.htaccess RewriteRule destroys CSS ( not working anymore )

Options +FollowSymlinks
RewriteEngine On

RewriteCond %{HTTP_HOST} ^domain.com [NC]
RewriteRule ^(.*)$ http://www.domain.com/$1 [R=301,NC]

RewriteRule ^([a-zA-Z0-9_-]+)$ $1.php
RewriteRule ^([a-zA-Z0-9_-]+)/$ $1.php

Code (markup):

Works:

http://www.domain.com/support

Code (markup):

Does NOT work ( no CSS ):

http://www.domain/support/

Code (markup):

What can I do to fix it and why does it do so ?

UPDATE: Solved by myself.